U.S. billionaire real estate mogul Donald Trump, the front-runner for the Republican presidential nomination, ridiculed his two rivals' joint effort to derail his campaign Monday, calling it a "horrible act of desperation."

Trump's challengers, Texas Senator Ted Cruz and Ohio Governor John Kasich, announced a plan late Sunday to try to deny the brash Trump, who has never held elective office, from winning the nomination on the first ballot at the party's July national convention, in hopes convention delegates will pick them on a subsequent ballot.

Kasich said he would stop campaigning in the midwestern state of Indiana, where Republicans vote in a nominating contest May 3, to give Cruz a better shot of winning there against Trump.

Meanwhile, Cruz agreed to halt his efforts in two western states, Oregon, which votes May 17, and New Mexico, with voting on June 7, to give Kasich more room to fight Trump for convention delegates in those states. Both Cruz and Kasich said they would continue to campaign against each other and Trump in other states.

Trump derided the Cruz-Kasich effort, saying, "It is sad that two grown politicians have to collude against one person who has only been a politician for 10 months in order to try and stop that person from getting the Republican nomination."

He said the two candidates' coordination to block him "would often be illegal" in corporate transactions."This horrible act of desperation from two campaigns who have totally failed, makes me even more determined, for the good of the Republican Party and our country, to prevail," he said.

Cruz said going head to head against Trump in Indiana, a conservative state with both rural expanses and industrial centers, will help voters there decide and also is "good for the country to have a clear and direct choice."

Kasich gave a mixed message about the coordination with Cruz, saying he would not spend "resources" in Indiana, but still called for people there to vote for him.

The Cruz-Kasich strategy signals a marked shift for Cruz, who has previously rejected overtures to join with Kasich to try to block Trump. The New York developer holds a commanding lead in convention delegates over both of his challengers, but is not yet assured of winning a majority of convention delegates before the quadrennial gathering starts.

Cruz has repeatedly called for Kasich to drop out, saying he was a spoiler in the race, winning votes and a handful of delegates that were denying Cruz a chance for a head-to-head faceoff with Trump.

Cruz, a conservative agitator against both Democratic and Republican leaders in Washington, has acknowledged that he cannot win the nomination on the first ballot, but says Trump, a one-time television reality show host, also won't reach the majority of 1,237 convention delegates needed to claim the nomination on the first round of voting. As its stands now, Trump has 845 pledged delegates on the first ballot, Cruz 559, Florida Senator Marco Rubio, who dropped out of the race a month ago, 171, and Kasich 148.

Trump, if he does well in the 15 remaining state nominating contests, could reach the 1,237 figure. He swept to victory in his home state of New York last week and is a heavy favorite over Cruz and Kasich in all five primaries scheduled Tuesday in five northeastern states, Pennsylvania, Maryland, Delaware, Connecticut and Rhode Island.

Trump leads pre-primary polls

U.S. political analysts view the vote in Indiana, a conservative state with both rural expanses and industrial centers, as another crucial step in deciding the outcome of the Republican nomination race. Pre-election surveys show Trump with a single-digit percentage lead over Cruz, but the outcome could tip with Kasich's withdrawal from the state, assuming his supporters don't stay home and actually switch their votes to Cruz rather than Trump.

Trump scoffed at both the Cruz and Kasich campaigns, saying that about 80 percent of the Republican voters have rejected Cruz and noting that Kasich has only won in Ohio, the Midwestern state he governs, while losing dozens of other contests.

Democratic presidential candidate Hillary Clinton meets with attendees during a campaign stop, April 24, 2016, at the University of Bridgeport in Bridgeport, Conn.

Democratic front-runner

The eventual Republican nominee is likely to face the Democratic front-runner, former Secretary of State Hillary Clinton, in the November national election to pick a successor to President Barack Obama, who leaves office in January. Clinton, seeking to become the first female U.S. president, holds a significant lead in Democratic convention delegates over her sole challenger, Vermont Senator Bernie Sanders, but has yet to seize a majority to claim the party's nomination.

Clinton, the wife of former President Bill Clinton, is favored in the Tuesday voting in all five states.